1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, to a cooling system which provides an essentially passive means of cooling the containment of a nuclear reactor following postulated design basis events. This cooling system, in conjunction with other plant features can also function as a passive ultimate heat sink by removing all core decay heat rapidly
2. Description of the Related Art
The removal of heat from the containment of a reactor following postulated accidents is a safety related function that is typically performed by two or more separate, redundant "trains" of mechanical components and cooling systems. In a water cooled reactor, each of these trains typically includes one or more fan coolers and a series of cooling systems, such as component cooling water, service water and an ultimate heat sink, all supported by AC power generated by diesel generators, electrical power cabling and switch gear, control circuitry, HVAC etc. Since all of these systems perform a safety related function, they must be seismically and environmentally qualified, and housed in seismic designed structures.
The aforementioned structures and approach to nuclear plant safety has resulted in plant designs which are highly complex and expensive, since significant construction and operational effort is associated with seismic design, qualification, testing, and maintenance of these buildings and components.
Passive systems have been devised in order to greatly reduce the number of safety grade components and eliminate the need for safety grade support system functions, including AC power, heating and air conditioning, large cooling water systems for component cooling and ultimate heat removal. These passive systems eliminate the need for operator actions, increase safety function reliability, and reduce risk to the public. Plant availability and reliability are improved, while complexity, maintenance, cost, and construction time are reduced.
A passive safety system is described in U.S. Pat. No. 4,753,771 to Conway et al. (one of the co-inventors herein) A system is described in which the amount of redundant safety grade support systems and their associated structures are reduced by maximizing the use of natural phenomenon, such as gravity, compressed gas, and natural circulation, to accomplish all safety functions.
The containment cooling system presented in the aforementioned patent is based on natural circulation air cooling of the outside of the steel containment. This concept, in order to be applicable to commercial sized reactor plants, would require a larger than typical containment surface area and/or higher than typical containment design pressure/temperature.
Generally, prior attempts to simplify the containment heat removal function has had several draw backs which have deterred their application. Specifically, passive containment cooling was not combined with other passive safety features so that significant reductions in safety related building and support systems could be made on a total plant basis. Thus, structures and/or components required for containment cooling would only have been viewed as cost additions. Since the frequency of events which require safety related containment cooling are very low, there would be no recognized favorable cost/benefit ratio.
Also, the desire to maximize economies of scale in nuclear plant applications has resulted in a large portion of plants with a rating of 2,700 megawatts thermal or greater. These large plants typically require containment shells having 130 to 150 foot diameters. In the past, the use of a cylindrical steel containment for this size range was not considered practical, and spherical steel containments were not in general use. Therefore, concrete containments were the typical design choice. These concrete containments cannot be externally surface cooled.
Surface cooling of a steel containment shell requires that the shield building surrounding the shell be open to the environment to provide an air inlet and outlet or to vent steam. This is contrary to current practice, where plants which have a steel containment utilize the shield building to create a containment annulus which is filtered and therefore acts as a second barrier to radiation release.
The use of water only on the outside of the containment shell to remove heat and limit containment pressure has practical limitations. First, if the containment shell outside surface is to be kept below 212.degree. F. (cooling water does not boil) a very large amount of cooling water is required. Providing this water using only gravity flow from an elevated tank is likely to be limited by practical storage capacity. Therefore, long term cooling would require additional qualified water sources, pumps, and associated structures such as piping and support systems. Thus, such a system would not be entirely passive. If the cooling water is to heat up significantly and boil, the outer shell surface temperature must be higher than 212.degree. F. Since the saturation temperature of steam/air at 40 psig (typical steel containment maximum transient pressure) is only 250 F., a higher containment design pressure would most likely be required to provide sufficient change in temperature for heat transfer.
The use of air only on the outside of the containment shell to remove heat and limit containment pressure also has practical limitations. For example, cooling a containment with only air would require a significant increase in the containment surface area over that provided in current reactor containment designs since the heat transfer rates achievable are relatively small.
The use of passive heat sinks, such as water or other materials, in continuous contact with the containment shell is likely to have a large impact on the containment structural design, and will create concerns about corrosion.